1. Field of the Invention
The present invention relates to an inkjet printing apparatus to eject and fly ink droplets through ink ejection openings of a print head to attach the ink droplets to a to-be-printed matter for printing. The present invention also relates to the reduction of factors causing the deterioration of an image such as an ejection defect.
2. Description of the Related Art
In recent years, printing apparatuses used for a printer, a copier, and a facsimile for example have higher performance requirements, including not only a high-speed printing and a full color printing but also a high-definition image equal to that by the silver halide photography. With regard to such requirements, an inkjet printing apparatus to eject ink for printing can eject minute ink droplets at a high frequency. Thus, the inkjet printing apparatus is superior in the high-speed printing and a high-quality printing to printing apparatuses using other printing methods.
Generally, the inkjet printing apparatus includes: a carriage (CR) including a print head and an ink tank; a transport means for transporting a printing medium; and a control device for controlling these components. According to this print method, a print head in which ink droplets are ejected through a plurality of minute ejection ports is subjected to the serial scan in a direction (main scanning direction) orthogonal to a printing medium transport direction (sub-scanning direction) and, when no print operation is performed, a printing medium is intermittently transported by an amount equal to a printing width. This printing method ejects ink on a printing medium depending on a printing signal. Thus, this printing method has been widely used as a method that achieves a low running cost and that is quiet.
However, in the case of the inkjet printing apparatus, when ink is allowed to fly through the respective ejection ports of the print head, the solvent in the ink evaporates over time depending on ink environment conditions near the respective ejection ports. Due to this, the neighborhood of an ejection port through which ink should be ejected first has ink of an increased viscosity and thus has a difficulty in ink ejection. If the circumstance as described above is combined with an extended time during which no ejection is performed because ink components having an increased density must be sucked and recovered without being exchanged, the next printing may be prevented from having a normal ejection to eject ink for printing. Regarding this, the maximum time during which a stable ejection can be guaranteed to the no-ejection time under a certain environment (temperature, humidity) (hereinafter also may be referred to as a stable ejection time) is used as one parameter showing the inkjet print head performance.
However, there has been recently a trend in which, in order to realize a further higher printing performance, ink is also required to provide a higher performance such as a higher color formation or high weather resistance. Thus, more inks include a higher amount of functional additives. However, the increased amount of these additives frequently causes an increased ink density or an increased density increase rate due to water evaporation, thus causing a disadvantage of a reduced stable ejection time. Furthermore, a further longer stable ejection time is required when the printing must be carried out with a long travel distance of the print head. For example, a longer stable ejection time is required with an increase of the travel of the print head in the main scanning direction from the A4 size through A3 size and A2 size to A1 size. In such a case, some inks or print heads may cause the stable ejection time to be shorter than the time required for one scanning (hereinafter referred to as a scan time).
Conventionally, several suggestions have been made to increase this stable ejection time by a nozzle structure of a print head, an ink composition, a main body mechanism, or a sequence. For example, there has been a conventional technique to reduce the time from a preliminary ejection to the printing by performing the preliminary ejection on a platen. However, the preliminary ejection can be performed only at limited positions on the platen from the viewpoints of the prevention of offsetting by avoiding ribs or the prevention of mist from being attached to a printing medium for example. Thus, this technique may not provide a sufficient effect. Another method is also considered by which the avoidance is achieved by increasing the travel speed of the print head in the main scanning direction.
In the case of this method however, image formation requires not only the increased travel speed of the print head but also a correspondingly-increased driving frequency at which ink is ejected through the print head. Thus, this method is limited from the viewpoints of the ejection pulse width and the ink refill speed for example.
In order to solve the disadvantage as described above, Japanese Patent Laid-Open No. 2007-144698 discloses a technique according to which an evaporation promotion mechanism is provided to humidify the neighborhood of the ejection port surface so that the stable ejection time can be increased. The evaporation promotion mechanism of Japanese Patent Laid-Open No. 2007-144698 is fixedly provided at the outer side of the platen. In this case, the humidification effect can be obtained only when the paper passes above the evaporation promotion mechanism, thus finding a difficulty in the optimization depending on the size of a printing medium. For example, in the case of a center paper feeding, some printing medium size requires a long distance between the printing region and the evaporation promotion mechanism. Thus, when the print head travels along this long distance, water in the ink normally evaporates, thus causing a disadvantage of a reduced humidification effect as a whole.
Furthermore, Japanese Patent Laid-Open No. 2007-144698 requires the evaporation promotion mechanism not directly contributing to the printing operation and the necessity for an ink tank for evaporation liquid, thus causing disadvantages such as an increased body size and an increased cost for example. Furthermore, when waste ink is used as evaporation liquid, the evaporation promotion mechanism can be provided only beside the home-side cap, thus failing to provide a humidification effect during a back scanning.